1. Field Of The Invention
This invention relates to a process for beneficiation of coal and an associated apparatus and more specifically, to the use of gaseous carbon dioxide or gaseous carbon dioxide mixed with air and amines in aqueous coal slurries to remove the undesired mineral matter from the "clean" coal.
2. Description Of The Prior Art
Coal is a vital and plentiful energy source. When coal is burned, however, sulfur dioxide is created. Sulfur dioxide, when mixed with rain water, causes acid rain. The undesirable effects of acid rain are numerous and well known. It is desirable to remove sulfur-bearing as well as ash forming mineral matter from the raw coal in order to use coal in a way that does not adversely effect the environment and coal conversion processes.
One known process for separating coal from mineral matter is set forth in U.S. Pat. No. 4,613,429. Raw coal which consists of "clean coal" and mineral matter is ground to ultrafine sizes (approximately 50 microns) so that the fine mineral matter in the raw coal can be separated from the clean coal. After this, a water slurry of the coal is introduced into a pressurized separation chamber containing liquid carbon dioxide. The liquid carbon dioxide and the liquid water form separate and distinct phases, with the liquid carbon dioxide forming an upper phase and the water forming a lower phase. The concentrated mineral content tends to remain as a slurry in the liquid water phase, while the coal tends to accumulate as a slurry in the liquid carbon dioxide phase.
U.S. Pat. No. 3,998,604 discloses the use of carbon dioxide as a coal flotation agent. This patent does not disclose the use of gaseous carbon dioxide in cooperation with a coal slurry containing amine for the separation of mineral matter from coal. In addition, carbon dioxide does not react during the flotation of coal.
U.S. Pat. No. 2,142,207 discloses a froth flotation process for cleaning coal. Gaseous carbon dioxide is bubbled upwardly through an aqueous slurry of coal to form a froth floating on top of the slurry. As coal has a lower specific gravity than the mineral matter, the coal is carried by the carbon dioxide bubbles to the top of the slurry and the mineral matter is left on the bottom.
In frothing or flotation technology it is desired to use small bubbles and to make the coal as hydrophobic as possible. Small bubble sizes increase the effectiveness of the flotation process because they more selectively capture fine coal particles. Also, because small bubbles (approximately 100-300 microns in diameter) are generally devoid of liquid wakes when moving upwardly through the aqueous slurry, less mineral matter is entrained along with these bubbles than if larger bubbles with associated large wakes are used. This increases the yield of the clean coal which is bubbled to the top of the slurry.
Small bubble sizes have been used in a process called Microbubble Flotation (MBF). See Luttrell, G. H., P. M. Keyser, T. T. Abel, and R. H. Yoon, "Improvements In Recovery And Selectivity With Microbubble Flotation Process", Proceedings Of The Second Annual Pittsburgh Coal Conference, Sept. 16-20, 1985, p. 43. Experimental evidence indicates that improved coal flotation can be attributed to the reduced turbulence behind the small bubbles. The liquid wakes formed behind small bubbles are generally absent and consequently mineral matter, which is hydrophillic in nature, is not entrained as the bubbles carrying the "clean coal" particles move from the bottom to the top of the flotation cell.
It is also desired to increase the hydrophobicity of the coal particles. This will increase the amount of clean coal that can be separated from the mineral matter. In a slurry, however, coal particles are surrounded by a liquid film, thus decreasing the amount of the exposed clean coal surface which in turn adversely affects the separation of the clean coal from the mineral matter.
One known method of shearing off or thinning the liquid film surrounding the clean coal particles and then exposing more of the clean coal's natural surface, is by creating turbulence in the slurry. This turbulence, however, creates large bubbles which have the associated problems discussed hereinabove. Furthermore, once bubble-particle attachment occurs, turbulence in the flotation cell is undesirable because subsequent detachment of the particles from the bubbles will occur. Thus, despite the benefits of turbulence, current MBF cells provide microbubbles which are introduced into the cell into a quiescent suspension at extremely low gas throughputs. Therefore, turbulence associated with large bubbles (approximately 500 microns in diameter) is virtually absent.
Another mode of increasing the hydrophobicity of coal particles is to use chemical agents, such as kerosene. See, Perry, P. H., Green, D., "Flotation", Chemical Engineer's Handbook, 6th Edition, pp. 21-46. These chemical agents increase hydrophobicity of coal by adsorption on the coal surface. These chemical agents, however, tend to have a deleterious effect on the performance of the process, because they tend to agglomerate coal. Agglomeration increases the effective particle size of the coal and thus, can impact the performance of frothing or flotation processes.
A related problem is maintaining a uniform dispersion of the microbubbles in the slurry. Though microbubbles are introduced in MBF cells, there is an inherent tendency towards the formation of larger bubbles in the cell. As bubbles rise to the surface in an aqueous slurry, they tend to grow in size due to decreased hydrostatic pressure. Also, because of the high bubble density associated with microbubbles, the liquid film between adjacent bubbles tends to collapse resulting in coalescence of a series of the smaller bubbles into larger bubbles. In aqueous slurries, the coalescence phenomena is further promoted due to the presence of solid particles. Use of surfactants prevents coalescence to a large extent but does not eliminate it.
In spite of these prior art teachings, there remains a need for an improved flotation process that will accomplish the separation of mineral matter from mineral ore in an effective manner.